Expand tests to reproduce #7655

The original `builtins.getContext` test from
1d757292d0cb78beec32fcdfe15c2944a4bc4a95 would have caught this. The
problem is that b30be6b450f872f8be6dc8afa28f4b030fa8d1d1 adding
`builtins.appendContext` modified that test to make it test too much at
once, rather than adding a separate test.

We now have isolated tests for both functions, and also a property test
showing everything put together (in the form of an eta rule for strings
with context). This is better coverage and properly reproduces the bug.

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